1. Field of the Invention
This invention relates to curable polyorganosiloxane compositions. More particularly, this invention relates to fluorinated polyorganosiloxane compositions that cure by a hydrosilation reaction to yield thermally stable gels exhibiting a resistance to solubilization in liquid hydrocarbons.
2. Description of the Prior Art
The term "polyorganosiloxane gel" typically defines a class of elastomeric, readily deformable materials exhibiting a surface hardness of below about 20 on the Shore 00 durometer scale. The hardness value of many gels is too low to be measured on this scale, and is often expressed as the depth to which a penetrometer probe of specified dimensions will penetrate or deflect the surface of the gel under a given loading.
U.S. Pat. No. 3,020,206, which issued to Nelson on Feb. 6, 1962, discloses organosiloxane gels that are reaction products of mixtures consisting essentially of an organosiloxane having repeating units of the formulae RViSiO, R.sub.2 SiO and CH.sub.3 R.sub.2 SiO.sub.0.5, and a liquid organohydrogensiloxane of the general formula HRCH.sub.3 SiO(R.sub.2 SiO).sub.n SiCH.sub.3 RH where each R individually represents a methyl or phenyl radical, Vi represents vinyl, the value of n is such that the viscosity of the material does not exceed 0.1 m.sup.2 /sec. at 25.degree. C., and at least 0.174 mole percent of the units in the organosiloxane are RViSiO. The mixture also includes a platinum catalyst and contains an average of from 1.4 to 1.8 gram atoms of silicon-bonded hydrogen per gram molecular weight of organosiloxane and at least one RViSiO unit for each silicon-bonded hydrogen atom. In accordance with the teaching of Nelson, an organosiloxane containing vinyl radicals at non-terminal positions is reacted with an organohydrogensiloxane containing silicon-bonded hydrogen atoms only at the terminal positions.
U.S. Pat. No. 4,072,635, which issued to Jeram on Feb. 7, 1978 teaches preparing organosiloxane gels from reactants similar to those disclosed in the aforementioned Nelson patent, with the exception that vinyl radicals and silicon-bonded hydrogen atoms can be located at both terminal and non-terminal positions in the organosiloxane and organohydrogensiloxane, respectively.
U.S. Pat. No. 4,374,967, which issued to Brown, Lee and Maxson on Feb. 22, 1983, describes dielectric silicone gels capable of remaining in the amorphous, non-crystalline phase at extremely low temperatures in the order of -120.degree. C. The gels are reaction products of(1) a polyorganosiloxane containing specified concentrations of monomethylsiloxy, dimethylsiloxy, trimethylsiloxy and dimethylvinylsiloxy units, (2) an organohydrogensiloxane and (3) a platinum catalyst.
The unique physical and chemical properties of polyorganosiloxane gels make them desirable as coatings and encapsulants for electrical and electronic devices to protect these devices from moisture, other contaminants and mechanical abuse, all of which can cause the device to malfunction or become inoperative.
Organosiloxane gels can also be used to coat or encapsulate printed circuit boards and other substrates on which are mounted electronic components such as solid state devices and integrated circuits. In some instances the circuit board is intended to be repairable by identifying and replacing defective component(s) rather than the entire circuit board. If the components and associated substrate on which the components are mounted were coated or encapsulated with a gel, the gel would have to be self-healing following removal of the probe of a voltmeter or other device used to identify the defective component(s). When this has been accomplished, a portion of the gel is cut away to permit replacement of the defective component, following which the new component is encapsulated with additional gel. To be useful in this application a gel must remain sufficiently clear to allow visual inspection of the components and soft enough to be cut away from the substrate to permit replacement of the defective component(s).
Electronic components associated with electronic ignition and emission control systems of automobiles and other vehicles powered by internal combustion engines are often located in the engine compartment of the vehicle where they are exposed to hydrocarbon fuels, lubricating fluids and temperatures of at least 100.degree. C.
The introduction of fluorinated hydrocarbon radicals into a polyorganosiloxane to impart resistance to swelling and/or solubilization by the liquid hydrocarbons present in gasoline and other fuels is disclosed in U.S. Pat. No. 2,979,519, which issued to Pierce et al. on Apr. 11, 1961 and in U.S. Pat. No. 3,719,619, which issued to Brown on Apr. 20, 1965.
One class of polyorganosiloxane gels is prepared by reacting a liquid polydiorganosiloxane containing silicon-bonded vinyl or other ethylenically unsaturated hydrocarbon radicals at each of the two terminal positions with a liquid organohydrogensiloxane containing an average of more than two silicon-bonded hydrogen atoms per molecule in the presence of a platinum catalyst. To achieve the physical properties characteristic of a gel the molar ratio of silicon-bonded hydrogen atoms to vinyl or other ethylenically unsaturated hydrocarbon radical is typically between 0.5 and 1.0.
Vinyl terminated polydiorganosiloxanes are often prepared by the polymerization of cyclic diorganosiloxanes containing an average of three or four R.sub.2 SiO units per molecule. In this formula each R individually represents a substituted or unsubstituted monovalent hydrocarbon radical that contains substantially no ethylenic unsaturation. The polymerization is conducted in the presence of a catalytic amount of an acidic or basic material. Suitable catalysts should be soluble in the reaction mixture and include acids such as sulfuric acid and the organosulfonic acids and basic materials such as the alkali metal hydroxides and alkali metal silanolates.
The production of hydroxyl terminated polydiorganosiloxanes containing from 2 to 250 or more repeating units per molecule by the reaction of cyclic diorganosiloxanes or linear polydiorganosiloxanes under superatmospheric pressure in the presence of ammonia as the catalyst is disclosed in U.S. Pat. No. 3,046,293, which issued to Pike on July 24, 1962.
If it is desired to prepare a diorganovinylsiloxy terminated polydiorganosiloxane using a relatively strong acid or base catalyst, a disiloxane of the general formula (R'.sub.2 ViSi).sub.2 O is included in the polymerization reaction mixture together with the cyclic diorganosiloxane. Alternatively the hydroxyl terminated polymer obtained from the polymerization of at least one cyclic diorganosiloxane is reacted with a hexaorganodisilazane of the general formula (R'.sub.2 ViSi).sub.2 NH. In the foregoing formulae R' represents a hydrocarbon radical free of ethylenic unsaturation and Vi represents a vinyl radical.
Prior art references typically disclose but do not exemplify gels formed by a hydrosilation reaction between a vinyl-containing fluorinated polyorganosiloxane and an organohydrogensiloxane. The present inventors found that when conventional non-volatile acidic or basic catalysts of the prior art are used to prepare diorganovinylsiloxy terminated polydiorganosiloxanes wherein at least about 75 mole percent of the repeating units contain a fluorinated hydrocarbon radical such as 3,3,3-trifluoropropyl bonded to silicon, the gels prepared by reacting these polymers with an organohydrogensiloxane often discolor and harden during relatively short exposures to temperatures of 100.degree. C. and above. Some gels discolor during curing.
U.S. Pat. No. 4,122,246, which issued to Sierawski on Oct. 24, 1978, teaches preventing the discoloration of gels during aging. The composition for preparing the gel contains (1) a polyorganosiloxane containing an average of about two vinyl radicals per molecule and an average of from 2 to 2.03 hydrocarbon or fluoroalkyl radicals per silicon atom; (2) an organosilicon compound having an average of at least 3 silicon-bonded hydrogen atoms per molecule; (3) a polysiloxane having at least one silicon-bonded hydroxyl radical per molecule, at least two silicon-bonded vinyl radicals per molecule, and an average of less than 15 silicon atoms per molecule; and (4) a silane having at least one silicon-bonded epoxy-substituted organo group, at least one silicon-bonded alkoxy group having less than 5 carbon atoms per group; and (5) a platinum catalyst. The combined weights of ingredients (3) and (4) are less than 1.5 weight percent of the composition and the molar ratio of silicon-bonded hydrogen atoms in (2) to vinyl radicals in (1) is less than 1. The gel obtained by curing the composition exhibits a penetration of from 2 to 60 millimeters measured using a Precision Universal Penetrometer. To achieve the desired resistance to discoloration ingredients (1), (3), and (4) must be combined before being blended with the remaining ingredients of the composition.
Following the teaching of Sierwaski to prepare a gel that is resistant to discoloration upon aging is less than desirable because of the two additional ingredients required, which increases the cost of the curable composition. In addition Sierwaski does not address the problem of hardening that has been observed when fluorinated polyorganosiloxane gels are heated at temperatures of 100.degree. C. and above.
One objective of this invention is to provide fluorine containing polyorganosiloxane compositions that can be cured to yield gels exhibiting a resistance to discoloration and hardening when heated, and to do so without requiring reactants other than a vinyl containing polydiorganosilxane, an organohydrogensiloxane and a curing catalyst.